What the FDA labels say about taking them together
Letermovir and phenytoin
Letermovir's FDA label of January 14, 2026 names phenytoin in its drug interactions section; the phenytoin labels PlainMeds reads (2) do not name letermovir. The letermovir label: “Co-administration of PREVYMIS and phenytoin is not recommended due to potential for loss of efficacy of PREVYMIS.”
Check it at the source: the letermovir FDA label on DailyMed (effective January 14, 2026, Merck Sharp & Dohme LLC); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the letermovir label say about phenytoin?
Letermovir label, drug interactionsConcomitant Drug Class and/or Clearance Pathway/Drug Name: phenytoin; Effect on Concentration ↓ =decrease, ↑ =increase: ↓ letermovir ↓ phenytoin; Clinical Comments: Co-administration of PREVYMIS and phenytoin is not recommended due to potential for loss of efficacy of PREVYMIS.
Source: FDA drug label (PREVYMIS), Merck Sharp & Dohme LLC, effective January 14, 2026 (SPL set 1b49df80-be4f-47e0-a0b7-123f3e69395b), via openFDA; the full label on DailyMed
What does the phenytoin label say about letermovir?
Phenytoin's label (effective October 15, 2024) and its 1 other brand-application label do not name letermovir in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Phenytek), Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Letermovir | Phenytoin | |
|---|---|---|
| Labeled for | n/a | Seizures and epilepsy |
| Boxed warning | No | No |
| Pharmacy cost (NADAC) | n/a | $0.145 per capsule 100 mg capsule |
| Shortage now | No current entry | No current entry |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once